This is the actual number of bytes written to the user memory pointed
to by 'sbp'. 'sb_len_wr' is always <= 'mx_sb_len'. Linux 2.2 series
kernels (and earlier) truncate this value to a maximum of 16 bytes.
The actual number of bytes written will not exceed the length
indicated by "Additional Sense Length" field (byte 7) of the Request
Sense response.
The type of sb_len_wr is unsigned char .